1. Field of the Invention
The present invention relates to a spot-size conversion optical waveguide.
2. Description of the Related Art
Both an optical fiber and a planar lightwave circuit (PLC) or the like as an optical waveguide circuit are optical waveguide elements forming a refractive index difference between a core portion and a cladding portion to confine a light within the core portion and making the confined light propagate therethrough. However, in many cases, a relative refractive-index difference Δ may differ between the optical fiber and the optical waveguide circuit. Therefore, spot sizes of lights propagating through the core portion may differ from each other in many cases. Specifically, a relative refractive-index difference Δ of an optical waveguide circuit is increased relative to a relative refractive-index difference Δ of an optical fiber for the purpose of down-sizing or the like sometimes. In a case where relative refractive-index differences Δ differ between the optical fiber and the optical waveguide circuit, spot sizes of propagating lights differ from each other. In this case, when the optical fiber is connected to the optical waveguide circuit optically in this state, a connection loss is produced in large degree due to mismatching of the spot sizes. In order to restrain such a large degree of connection loss from being produced, a spot-size conversion optical waveguide is disclosed which is disposed at a position where the optical fiber is connected to the optical waveguide circuit to convert the spot size of a light (for example, see Japanese Patent Application Laid-open Publication No. 2007-093743, Japanese Patent Application Laid-open Publication No. 2011-022464 and Tokushima et al., “Dual-Tapered 10-μm-Spot-Size Converter with Double Core for Coupling Polarization-Independent Silicon Rib Waveguides to Single-Mode Optical Fibers”, Applied Physics Express 5 (2012) 022202). A spot-size conversion optical waveguide as such is used as well in a case of connecting optical waveguide circuits of which spot sizes of lights differ from each other.